This invention relates to an intrauterine pulse oximetry apparatus and method for measuring fetal oxygen saturation during labor and delivery. The invention particularly relates to placing the apparatus within a preferred region on the fetus.
A fetal pulse oximetry apparatus may include a sensor attached to an electrical cable. The sensor, which contains a light source and a light detector, is placed on the fetus. The cable connects the sensor to a pulse oximeter. Light from the light source is transmitted through the fetal tissue and reflected back to the light detector. The amount of light received by the light detector depends on characteristics of the blood in the fetal tissue, among other things.
Fetal pulse oximetry sensors are not new. Some previous apparatus were physically attached to the fetal skin by hooks, spirals, suction, or glue. One example is the fetal pulse oximetry sensor disclosed in PCT Publication No. WO 90/01293. These attachment means were invasive or potentially harmful. Therefore the apparatus were placed only in fetal regions that the doctor or other user could reach with his or her fingers, such as on the presenting part of the fetus or on the fetus within the uterine region just beyond the cervix (the xe2x80x9ctranscervical regionxe2x80x9d).
The structure of previous apparatus did not permit safe and accurate placement on the fetus in the region beyond the reach of the user. In addition, nothing was known of fetal pulse characteristics in this region. Thus placement of the apparatus beyond the presenting part or in the transcervical region was considered undesirable.
Our tests have shown that fetal pulses are stronger in the region on the fetus beyond the presenting part and beyond the transcervical region. We call this fetal region the xe2x80x9cpreferred region.xe2x80x9d The present invention monitors pulses in the preferred region.
In a vertex presentation (the fetus descending headfirst), cervical pressure on the presenting part creates local edema (caput) which can suppress the fetal pulse and make pulse oximetry readings unreliable. The amplitude of the pulse in the presenting part also will change as the cervix dilates and changes the local force on the vertex.
During the periodic contractions of the uterine wall, additional local forces on the presenting part of the fetus are exerted actively by the cervix and passively by the pelvic bones. These transient local forces may further affect pulse amplitude. Thus, obtaining strong and consistent pulses throughout labor and delivery may be difficult.
The readings also may be affected by fetal hair. Depending on its color and amount, hair attenuates the light to various extents. Hair also may cause light to be shunted from the light source to the light detector, which adversely affects the measurement.
The present invention overcomes some of the shortcomings of previous fetal pulse oximetry apparatus and their placement methods. It provides an apparatus and a method of placing it in the preferred region, i.e. on the fetus beyond the presenting part and beyond the transcervical region. Pulse amplitudes in the preferred region are not affected by caput. They are less affected by cervical and pelvic bone forces than in the presenting part or the transcervical region. Since there is less hair (or even no hair) in the preferred region, the light transmission to and from the fetal tissue will be less attenuated and less susceptible to shunting. The apparatus and method allow the user to place the sensor without damaging the sensitive fetal eyes and fontanelles.
The preferred embodiment of the apparatus provides an electrical cable having a stiffer part adjacent to the sensor. This stiff part of the cable can be used to guide the sensor into position. The stiff part of the cable is rigid enough to be guided through the vagina and cervix without an introducer. It is flexible enough to yield when the sensor encounters an obstruction such as the uterine wall. With the sensor in position, the stiff part of the cable bends around the fetal head and conforms to the mother""s pelvic curve. This conformance allows prolonged application of the sensor without discomfort to the mother.
The stiff part of the cable has calibrated visual markings and one or more tactile markings (ridges). The visual markings are particular distances from the leading edge of the sensor. The ridge is located at a position approximating the distance from the vertex of the fetal head to a site well within the preferred region on a fetus at term. The ridge may coincide with one of the visual markings.
Devices inserted into the mother""s uterus must be sterile to avoid infection. As in prior art, the user must manipulate the apparatus within the vagina to place the sensor. If a prior-art device failed, the user would have to perform an additional vaginal examination to remove it. It would have to be sterilized before being reapplied. It also could be reapplied only with an introducer. If sterilizing were impractical, a new device would have to be used.
The current apparatus overcomes these limitations. If the sensor does not perform properly in its initial placement, the user can grasp the exposed stiff part of the cable and insert or withdraw it a small amount, for example 1 cm, as indicated by the visual markings. This action will reposition the sensor on a new site within the preferred region of the fetal head without removing the sensor from the uterus.
The method of using the apparatus has many advantages over prior art. It does not reintroduce anything into the vagina. Pulse oximetry readings thus can be taken without re-sterilizing the apparatus, outfitting it with an introducer, or using a new apparatus. The method typically is painless for the mother: it does not require uncomfortable manipulation of the apparatus inside the vagina. The method can be repeated during labor as necessary, for optimal sensor performance.
With the stiff part of the cable the user also can monitor the station of the fetal head (the position of the fetal head within the mother""s pelvis). Fetal head station is important to evaluate the progression of labor. It is determined by internal examination, which assesses the position of the fetal head relative to the pelvic spines. The apparatus and method of the current invention permit the user to continuously assess this position without frequent repeated internal examinations.
As the fetal head descends during labor, the sensor descends with it. If the user notes the station of the fetal head when the sensor is initially placed, the station can be correlated with the amount of the stiff part of the cable protruding from the vagina. The visual markings indicate how much of the stiff part is exposed. The exposed stiff part of the cable will represent the station of the fetal head.
One skilled in the art will recognize that the apparatus could operate as a fetal station indicator without using the oximetry sensor so long as some means is provided of placing the inner end of the tube against the fetus within a preselected region. A trailer extending from the inner end of the indicator through the mother""s vagina would have visual markings correlating to the station of the fetal head.
The objects of the invention are as follows:
To obtain improved pulse oximetry readings by measuring fetal oxygen saturation in a region with stronger pulses.
To obtain improved pulse oximetry readings by measuring fetal oxygen saturation in a region less affected by caput, cervical pressures, and hair.
To obtain the improved readings safely, that is without injuring the fetus or the mother.
To obtain the readings comfortably for the mother.
To allow prolonged yet comfortable and accurate monitoring of the fetus.
To orient the apparatus effectively without seeing or ultimately feeling the sensor, since the sensor""s target is beyond the reach of the user.
To reposition the sensor in the preferred region without withdrawing the apparatus from the uterus.
When repositioning, to avoid re-sterilizing the apparatus.
When repositioning, to avoid using an introducer.
When repositioning, to avoid using a new (sterile) apparatus.
When repositioning, to avoid causing discomfort to the mother.
To determine the station of the fetal head during labor without repeated internal examinations.